The present invention relates to a a device for detecting an tracking error of an optical head for optically recording and reproducing information, and particularly a tracking error detection device which produces little offset and in which the detection sensitivity has little dependence on the tracking pitch.
FIG. 8 shows the configuration of a conventional tracking error detection device using a push-pull method, shown in Japanese Patent Kokoku Publication No. 1412/1992 and "G. Bouwhuis et al., Principles of Optical Disc System, Adam Hilger, pp.72 to 73 (1985)." In the figure, reference numeral 1 denotes a light source such as a semiconductor laser, which emits a light beam for recording and reproduction. Reference numeral 2 denotes a collimator lens for converting the light beam from the light source 1 into a parallel beam. Reference numeral 3 denotes a beam splitter which passes the parallel beam from the collimator lens 2 and reflects the beam reflected at an information recording medium 5, which will be described later. Reference numeral 4 denotes an objective lens which focuses the light beam emitted from said light source 1 onto the information recording surface 6 of the information recording medium 5, such as an optical disk, and converts the beam reflected by the information recording medium 5 into a parallel beam. Reference numeral 7 denotes a light spot formed on the information recording surface 6 of the information recording medium 5. Reference numeral 8 denotes a track, which is parallel to the x direction, as shown in the figure. The y direction is perpendicular to the track 8 and is in a plane parallel to the information recording medium 5. The z direction is perpendicular to the information recording surface 6. Reference numeral 9 is a converging lens for concentrating a reflected beam in a suitable area on a split photodetector 10. The split photodetector has two light-receiving faces 11 and 12. Reference numeral 100 denotes a light spot on the split photodetector 10. The differential amplifier 13 produces a difference between the outputs from the light-receiving faces 11 and 12 as a tracking error signal TES. The tracking error signal TES is supplied via the phase-compensation circuit/amplifier 14 to the objective lens driving mechanism 15.
The operation of the conventional tracking error detection device shown in FIG. 8 will next be described. The light spot 100 on the split photodetector 10 is circular as shown in FIG. 8. The split photodetector 10 is disposed so that the boundary between the light-receiving faces 11 and 12 divides the circular spot into upper and lower halves. When the light spot 7 travels along the track 8 at its center, the amount of light received by the light-receiving face 11 equals to the amount of light received by the light-receiving face 12. When the light spot 7 deviates from the center of the track 8, the amount of light received at the light-receiving face 11 is different from the amount of light received at the light-receiving face 12. The direction (right or left) in which the light spot 7 deviates from the center of the track 8 determines whether the difference in amount of received light between the light-receiving faces 11 and 12 is positive or negative. Consequently, the difference in output between the light-receiving faces 11 and 12 can be treated as a tracking error signal.
The lateral movement of the objective lens 4 to the position indicated by a dotted line causes the lateral movement of the light spot 100 to the position indicated by a dotted line on the split photodetector 10, as shown in FIG. 8. Consequently, the light-receiving faces 11 and 12 receive different amounts of amount of light even if the light spot 7 is at the center of the track 8.
When the information recording medium 5 tilts relative to the y direction, the light spot 100 on the split photodetector 10 deviates. Consequently, the light-receiving faces 11 and 12 receive different amounts of light even if the light spot 7 is at the center of the track 8.
As a solution to these problems, Japanese Patent Kokoku Publication No. 34212/1992 describes a method in which two light spots are disposed on an information recording medium, with a spacing of about a half of the track pitch; the beams returning from the two light spots are received by respective two split photodetectors; and the difference between the differential outputs of the two split photodetectors is treated as a tracking error signal. The configuration, operation, and problems of this method will next be described, with reference to FIGS. 9 and 10.
FIG. 9 is a perspective view showing the configuration of another conventional tracking error detection device shown in Japanese Patent Kokoku Publication No. 34212/1992. In the figure, reference numerals 16 and 17 denote light sources such as a semiconductor laser, from which light beams with different wavelengths are emitted (the oscillation wavelength from the light source 16 is represented by .lambda.1 while the oscillation wavelength from the light source 17 is represented by .lambda.2). Reference numerals 18 and 19 denote collimator lenses for converting light beams from the light sources 16 and 17 into parallel beams. Reference numeral 20 denotes a beam splitter which deflects the parallel beam with wavelength .lambda.1 output from the collimator lens 18 by 90 degrees and directs the beam to the objective lens 4. Reference numeral 21 denotes another beam splitter which deflects the parallel beam with wavelength .lambda.2 output from the collimator lens 19 by 90 degrees and directs the beam to the objective lens 4. The beam splitters 20 and 21 in combination serve to merge the light beams from the light sources 16 and 17. Reference numerals 22 and 23 denote light spots formed on the information recording surface 6 of the information recording medium 5. The light spot 22 is of the light beam having wavelength .lambda.1 while the light spot 23 is of the light beam having wavelength .lambda.2.
The two light beams reflected by the information recording medium 5 are re-converted into parallel beams by the objective lens 4. The beams pass through the beam splitters 20 and 21, then reaches the dichroic beam splitter 24. The dichroic beam splitter 24 passes the light beam with wavelength .lambda.1 and reflects the light beam with wavelength .lambda.2. The beam splitter 24 thus divides the light beams from the information recording medium 5 into the light beam having the wavelength .lambda.1 and the light beam having the wavelength .lambda.2. Reference numerals 25 and 28 denote split photodetectors, which have two light-receiving faces 26 and 27, and 29 and 30 respectively. The beam returning from the light spot 22 of the light beam with wavelength .lambda.1 passes through the dichroic beam splitter 24 and reaches the split photodetector 25. The beam returning from the light spot 23 of the light beam with wavelength .lambda.2 is reflected by the dichroic beam splitter 24 and reaches the split photodetector 28.
FIG. 10 shows the relative positions of the light spots 22 and 23 on the information recording surface 6, the corresponding light spots on the split photodetectors 25 and 28, and a circuit for generating a tracking error signal. The figure shows that the information recording surface 6 of the information recording medium 5 has grooves and lands on it and that the tracks 8 are formed on the lands. The two light spots 22 and 23 are disposed with a spacing of p/2 in the y direction (direction perpendicular to the track), where p is the track pitch. Reference numerals 31 and 32 denote the light spots corresponding to the light spots 22 and 23 respectively on the split photodetectors. The outputs from the two light-receiving faces 26 and 27 of the single split photodetector 25 are input to the differential amplifier 33, from which differential output TE1 is obtained and supplied to the next differential amplifier 36. The outputs from the two light-receiving faces 29 and 30 of another split photodetector 28 are input to the differential amplifier 34, from which differential output TE2 is obtained. TE2 is supplied via the variable-gain amplifier 35 having gain G to the differential amplifier 36. The differential amplifier 36 outputs a difference between TE1 and TE2 multiplied by G, which is treated as tracking error signal TES. The tracking error signal TES is supplied via the phase-compensation circuit/amplifier 14 to the objective lens driving mechanism 15.
Next, it will be briefly described that the tracking error signal TES is free from the offset caused by the lateral movement of the objective lens. As described above, the offset results from the lateral movement of the spots on the split photodetector in the same direction caused by the lateral movement of the objective lens. In FIG. 10, the two spots 31 and 32 move in the y direction. The light-receiving faces 26 and 29 receive a greater amount of light than the light receiving faces 27 and 30. This causes a positive offset in TE1 and TE2, as indicated by a chain line in FIG. 11. When the optical disk is eccentric relative to the axis of rotation, the track crossing component of the tracking error signal caused by a light spot crossing the tracks 8 on the time base varies in a sinusoidal fashion, whose time period corresponds to the time while the optical disk makes one revolution. Because the spacing between the light spots 22 and 23 in the y direction is just a half of the track pitch, the differential outputs TE1 and TE2 have opposite phases, as shown in FIG. 11.
When the gain G of the variable-gain amplifier 35 is adjusted to the ratio of the offset of the differential output TE1 to the offset of the differential output TE2, the offset can be eliminated from the tracking error signal TES, as shown in FIG. 11. The offset caused by a tilt of an information recording medium can be eliminated in the same way.
Since the latter conventional tracking error detection device is configured as described above, the amplitude of the tracking error signal depends on the ratio of the spacing between the two light spots to the track pitch of the information recording medium. The dependence causes a problem especially when a single optical head is used to reproduce optical disks of different types with different track pitches. For example, let us assume that an optical head is adjusted so that the spacing s between the light spots is half the track pitch of a first type of optical disk having a track pitch p1, i.e., s=p1/2, so that the amplitude of the tracking error signal is maximized. When this optical head is used with a second type of optical disk having a track pitch p2, which is about half the track pitch p1, the spacing s between the light spots is about equal to the track pitch p2, and resultant amplitude of the tracking error signal will be almost zero. This is because, in this method, the amplitude of the tracking error signal is maximized when the spacing between two light spots is an odd multiple of a half of the track pitch, and the amplitude is zero when the spacing is an integral multiple of the track pitch.